1. Field of the Invention
The present invention relates to an image forming apparatus and a fixing device installed therein that use a copier, a printer, and a facsimile or an electrophotographic method of a multifunction product (MFP) such as the copier, the printer, and the facsimile.
2. Description of the Related Art
In an image forming apparatus such as a copier and a printer, a commonly used fixing device in which a nip is formed by pressure welding a fuser roller and a pressure roller conveys to the nip, a recording medium that bears a toner image and fixes the toner image on the recording medium (for example, see Japanese Patent No. 3506135).
In such a fixing device, for shortening a warm-up time of a device by reducing a heat capacity, as a fuser roller and the pressure roller, a hollow structured roller member having a thin elastic layer on a core bar is widely used. Usually in the fuser roller, in addition to the core bar (a core bar layer) and the elastic layer, a releasing layer is formed on a surface.
In Japanese Patent Application Laid-Open No. 3506135, a technology is disclosed in which a supporting member is press-fit inside the fuser roller for enhancing a dent strength margin of the thin-walled hollow structured fuser roller.
Furthermore, in Japanese Patent Application Laid-Open No. 2002-318502, a technology is disclosed in which a pushing roller is installed inside the hollow structured pressure roller for preventing generation of wrinkles on the recording medium and a radius and a deflection value of a central portion of the pushing roller and a deflection value of a central portion of the fuser roller (heating roller) is optimized.
However, by using the commonly used fixing device, the warm-up time cannot be shortened, a good fixability, and a high-longevity cannot be obtained in a simple structure.
The warm-up time is a time required for a surface temperature of the fuser roller to reach up to a temperature necessary for a fixing process. The good fixability is obtained when a uniformity between a nip width (nip measurement) of the nip in a convey direction and the nip width in a rotating axial direction (a direction that is orthogonal to the convey direction) is adequately secured. The high-longevity can be obtained when a bonding between the core bar and the elastic layer is remained intact or a big deformation is not formed on the core bar and the elastic layer.
The inventor of the present application has realized after an investigation that the core bar structure of the fuser roller and the pressure roller that are considered as the roller members is significantly related to three features that are mentioned earlier.
For example, in the roller member, if an outer diameter of the core bar is large, even if the good nip width can be maintained, a long warm-up time is required. If the outer diameter of the core bar is small, even if the warm-up time can be shortened, the good nip width cannot be maintained. Furthermore, if the core bar is thick, even if the high-longevity can be obtained, the long warm-up time is required and the good nip width cannot be maintained. If the core bar is thin, even if the warm-up time can be shortened and the good nip width can be maintained, the high-longevity cannot be obtained.
In the technology disclosed in Japanese Patent No. 3506135, because the supporting member is press-fit in the thin-walled hollow structured fuser roller, the dent strength margin of the fuser roller is enhanced and a load bearing capacity of the fuser roller, and a speed of the fixing device can be increased. However, the structure of the fuser roller becomes complicated by press fitting the supporting member, thereby not enabling to directly resolve the problem mentioned earlier.
In the technology that is disclosed in Japanese Patent Application Laid-Open No. 2002-318502, by optimizing the radius and the deflection value of the central portion of the pushing roller that is installed inside the pressure roller and the deflection value of the central portion of the fuser roller (heating roller), generation of wrinkles on the recording medium are prevented. However, the structure of the fixing device becomes complicated due to installation of the pushing roller, thereby not enabling to directly resolve the problem mentioned earlier.